1. Field of the Invention
The present invention relates to a braking system for use in an automotive vehicle and particularly to a hydraulic braking system comprising a tandem master cylinder and a dynamic hydraulic braking pressure generator which outputs hydraulic power pressure supplied from a power source in response to the depression of a brake pedal.
2. Description of the Prior Art
In a conventional service braking system for an automotive vehicle, there are provided a plurality of hydraulic circuits connecting wheel brake cylinders mounted on road wheels with a hydraulic braking pressure generator such as a master cylinder, so that when one of the hydraulic circuits is broken, the braking operation is achieved by the rest of the hydraulic circuits. In general, a tandem master cylinder is used in conventional dual circuits system.
In order to reduce the force required to operate a brake pedal in the braking operation, the hydraulic braking system is provided with a servo unit which is referred to as a servo or a booster and which utilizes compressed air, intake manifold vacuum (for a vacuum booster), or hydraulic pressure (for a hydraulic booster) as a power source. The hydraulic booster is a booster which actuates the hydraulic braking pressure generator such as the master cylinder by the hydraulic power pressure supplied from the power source in response to depression of the brake pedal. For example, the Japanese patent Laid-open publication No. 59-209948 discloses a system providing the hydraulic booster to the tandem master cylinder which will operate as the ordinary tandem master cylinder when the hydraulic booster is not operated.
With employment of such a hydraulic booster, it has been proposed to employ the hydraulic generated as a dynamic hydraulic pressure generator in addition to the master cylinder. In other words, a hydraulic pressure boosted by the booster (hereinafter referred to as the boost pressure) in response to the depression from the brake pedal is applied directly to a hydraulic circuit. For example, as shown in Japanese Patent Laid-open publication No. 59-227552, boost pressure of the hydraulic booster is applied to rear wheel brake cylinders in a front-rear dual circuits system in order to reduce the stroke of the brake pedal.
Further, regarding the hydraulic braking pressure generator provided with the tandem master cylinder and the hydraulic booster, it has been proposed to connect one of the pressure chambers of the tandem master cylinder with the wheel cylinders of one of the hydraulic circuits and to output the boost pressure of the hydraulic booster to the wheel cylinders of the other hydraulic circuits via the other pressure chamber of the tandem master cylinder. Such a hydraulic braking system is disclosed in Japanese Patent Laid-open publication No. 62-155167. According to this system, the various effects such as the shortening of the stroke of the brake pedal and so on are obtained.
In the above described prior art, (Japanese Patent Laid-open publication No. 59-227552), however, since the boost pressure of the hydraulic booster, namely the dynamic hydraulic pressure is directly supplied to the rear wheel cylinders, if the loss is generated at the rear wheel cylinders or the hydraulic conduit connecting with the rear wheel cylinders, there is a danger of the flow of the brake fluid filling up the power pressure circuit, including the hydraulic booster. And then, in the prior art, (Japanese Patent Laid-open publication No. 62-155167), the boost pressure, namely the dynamic hydraulic pressure is directly supplied to the wheel cylinders of the other hydraulic circuits. Accordingly, the flow of the brake fluid in the normal side is generated by the communication between the wheel cylinders and the dynamic hydraulic braking pressure generator when either of the wheel cylinder or the dynamic hydraulic braking pressure generator is lost.